Methods for synthesis of high quality carbon single-walled nanotubes

ABSTRACT

Methods and processes for synthesizing high quality carbon single-walled nanotubes (SWNTs) are provided. A carbon precursor gas is contacted with a catalyst deposited on a support. The temperature and the duration of the reaction are varied to produce SWNTs with the ratio of G-band to D-band in Raman spectra (I G :I D ) of about 5 to about 70.

FIELD OF INVENTION

The present invention relates to methods for the synthesis of carbon single-walled nanotubes using chemical vapor deposition method.

BACKGROUND

Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.

Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale. Present methods produce both single-walled and multi-walled carbon nanotubes, as well as other contaminants, and purification of SWNTs can be time consuming and expensive.

Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.

A method for the production of SWNTs is disclosed in U.S. Pat. No. 6,333,016 to Resasco et al. The catalyst is a bimetallic having one metal from Co, Ni, Ru, Rh, Td, Ir, or Pt, and one metal from Mo, W, or Cr. The bimetallic catalyst is deposited on a support and used for the synthesis of SWNTs using CO as the carbon source. The method can produce SWNTs on a commercial scale.

It is generally recognized that smaller catalyst particles of less than 3 nm are preferred for the growth of smaller diameter carbon nanotubes. However, the smaller catalyst particles easily aggregate at the higher temperatures required for the synthesis of carbon nanotubes. U.S. Patent Application No. 2004/0005269 to Huang et al. discloses a mixture of catalysts containing at least one element from Fe, Co, and Ni, and at least one supporting element from the lanthanides. The lanthanides are said to decrease the melting point of the catalyst by forming alloys so that the carbon nanostructures can be grown at lower temperatures.

Aside from the size of the catalyst, the temperature of the reaction chamber can also be important for the growth of carbon nanotubes. U.S. Pat. No. 6,764,874 to Zhang et al. discloses a method of preparing nanotubes by melting aluminum to form an alumina support and melting a thin nickel film to form nickel nanoparticles on the alumina support. The catalyst is then used in a reaction chamber at less than 850° C. U.S. Pat. No. 6,401,526, and U.S. Patent Application Publication No. 2002/00178846, both to Dai et al., disclose a method of forming nanotubes for atomic force microscopy. A portion of the support structure is coated with a liquid phase precursor material that contains a metal-containing salt and a long-chain molecular compound dissolved in a solvent. The carbon nanotubes are made at a temperature of 850° C.

The methods described above still produce significant by-products and/or low yields of SWNTs. Thus, there is a need for methods and processes for controllable synthesis of high quality carbon single walled nanotubes. Accordingly, the present invention provides novel methods and processes for the synthesis of high quality SWNTs at low reaction temperatures.

SUMMARY

The present invention provides methods and processes for growing single-walled carbon nanotubes in high yields and with high quality.

The methods use catalysts selected from a Group V metal, a Group VI metal, a Group VII metal, a Group VIII metal, a lanthanide, or a transition metal or combinations thereof. The catalyst preferably has a particle size between about 0.1 nm to about 10 nm. The catalyst can be supported on a powdered oxide, such as Al₂O₃, SiO₃, MgO and the like, wherein the catalyst and the support are in a ratio of about 1:1 to about 1:50. The supported catalyst can be used for the synthesis of SWNTs. The reaction temperature, the reaction duration, and other reaction conditions can be altered until high-quality SWNTs are produced and the production of other carbon species, such as multi-walled carbon nanotubes, amorphous carbon, disordered carbon, or graphite is minimized.

In one aspect, the invention provides methods for synthesizing carbon single-walled nanotubes (SWNTs) wherein supported metal catalyst are contacted with a carbon precursor gas and SWNTs are synthesized with a ratio of G-band to D-band in Raman spectra (I_(G):I_(D)) of greater than 5.

In another aspect, the invention provides methods for synthesizing carbon single-walled nanotubes (SWNTs) wherein metal catalyst on a support in a ratio of about 1:10 to about 1:20 are contacted with a carbon precursor gas and SWNTs are synthesized with a ratio of G-band to D-band in Raman spectra (I_(G):I_(D)) of greater than 5, and wherein the metal catalyst is selected from the group consisting Fe—Mo, Fe—Co and Ni—Fe—Mo.

In another aspect, the methods involve contacting a carbon precursor gas with a catalyst on a support, wherein the catalyst has an average diameter of less than about 2 nm and wherein high quality SWNT are formed. The average particle size of a metal catalyst by obtaining magnetization curve of the metal catalyst using SQUID and determining the average particle size from the magnetization curve. The carbon precursor gas can be methane that can additionally contain other gases such as argon and hydrogen. The catalyst can be a Group V metal, a Group VI metal, a Group VII metal, a Group VIII metal, a lanthanide, or a transition metal or combinations thereof. The catalyst preferably has a particle size between about 1 nm to about 50 nm. The catalyst can be supported on a powdered oxide, such as Al₂O₃, SiO₃, MgO and the like, where the catalyst and the support are in a ratio of about 1:1 to about 1:50.

In another aspect, the invention provides a carbon nanotube structure produced by the process of contacting a carbon precursor gas with a catalyst on a support at a temperature below about 750° C. The carbon precursor gas can be methane that can additionally contain other gases such as argon and hydrogen. The catalyst can be a Group V metal, a Group VI metal, a Group VII metal, a Group VIII metal, a lanthanide, or a transition metal or combinations thereof. The catalyst preferably has a particle size between about 1 nm to about 15 nm. The catalyst can be supported on a powdered oxide, such as Al₂O₃, SiO₃, MgO and the like, wherein the catalyst and the support are in a ratio of about 1:1 to about 1:50. The reaction duration can be about 30 min to about 90 min. The reaction temperature and the reaction duration can be altered such that high quality SWNTs are synthesized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts Raman spectra of carbon SWNTs grown using the optimized catalyst (Fe:Mo:Al₂O₃ at a ratio of 1:0.2:16.9), heat treatment and SWNT synthesis duration.

DETAILED DESCRIPTION

I. Definitions

Unless otherwise stated, the following terms used in this application, including the specification and claims, have the definitions given below. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Definition of standard chemistry terms may be found in reference works, including Carey and Sundberg (1992) “Advanced Organic Chemistry 3^(rd) Ed.” Vols. A and B, Plenum Press, New York, and Cotton et al. (1999) “Advanced Inorganic Chemistry 6^(th) Ed.” Wiley, New York.

The terms “single-walled carbon nanotube” or “one-dimensional carbon nanotube” are used interchangeable and refer to cylindrically shaped thin sheet of carbon atoms having a wall consisting essentially of a single layer of carbon atoms, and arranged in an hexagonal crystalline structure with a graphitic type of bonding.

The terms “metalorganic” or “organometallic” are used interchangeably and refer to co-ordination compounds of organic compounds and a metal, a transition metal or a metal halide.

II. Overview

The present invention discloses methods and processes for the synthesis of high quality carbon single-wall nanotubes (SWNT) and structures using the carbon vapor deposition method. Catalyst supported on powdered Al₂O₃ is used for the synthesis of SWNTs. The catalyst preferably is Fe and at least one other metal, such as Mo, Ni, and the like. The temperature of the reaction is selected such that the formation of amorphous or disordered carbon is minimized and the duration of the reaction is selected such that the formation of multi-layered carbon is minimized. Thus, high quality SWNTs are produced.

III. The Catalyst

The catalyst composition can be any catalyst composition known to those of skill in the art. Conveniently, the catalyst can be a metal or an alloy, such as, for example, iron, iron oxide, molybdenum, or a ferrite such as cobalt, nickel, chromium, yttrium, hafnium or manganese. The catalyst useful according to the invention will preferably have an average overall particle size of up to 50 nm to about 1 μm, although, in general, the particle sizes for individual particles can be from about 400 nm to about 1 μm.

The function of the catalyst when used in the carbon nanotube growth process is to decompose the carbon precursors and aid the deposition of ordered carbon. The methods and processes of the present invention preferably use metal nanoparticles as the metallic catalyst. The metal or combination of metals selected as the catalyst can be processed to obtain the desired particle size and diameter distribution. The catalyst can then be separated by being supported on a material suitable for use as a support during synthesis of carbon nanotubes as described below. As known in the art, the support can be used to separate the catalyst particles from each other thereby providing the catalyst materials with greater surface area in the catalyst composition. Such support materials include powders of crystalline silicon, polysilicon, silicon nitride, tungsten, magnesium, aluminum and their oxides, preferably aluminum oxide, silicon oxide, magnesium oxide, or titanium dioxide, or combination thereof, optionally modified by additional elements, are used as support powders. Silica, alumina and other materials known in the art may be used as support, preferably alumina is used as the support.

The catalyst can be selected from a Group V metal, such as V or Nb, and mixtures thereof, a Group VI metal including Cr, W, or Mo, and mixtures thereof, VII metal, such as, Mn, or Re, Group VIII metal including Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, and mixtures thereof, or the lanthanides, such as Ce, Eu, Er, or Yb and mixtures thereof, or transition metals such as Cu, Ag, Au, Zn, Cd, Sc, Y, or La and mixtures thereof. Specific examples of mixture of catalysts, such as bimetallic catalysts, which may be employed by the present invention include Co—Cr, Co—W, Co—Mo, Ni—Cr, Ni—W, Ni—Mo, Ru—Cr, Ru—W, Ru—Mo, Rh—Cr, Rh—W, Rh—Mo, Pd—Cr, Pd—W, Pd—Mo, Ir—Cr, Pt—Cr, Pt—W, and Pt—Mo. Preferably, the metal catalyst is iron, cobalt, nickel, molybdenum, or a mixture thereof, such as Fe—Mo, Fe—Ni and Ni—Fe—Mo.

The ratio of each metal to the total catalyst is preferably from about 1:10 to about 15:1 (mol/mol), more preferably about 1:5 to about 5:1 (mol/mol), and even more preferably about 1:2 to about 1:4 (mol/mol). Thus, for example, if the catalyst is the bimetallic Fe—Mo, the ratio of Fe:Mo can be 1:1, 2:3, 1:2, 3:2, 5:1, 6:1 and the like. Similarly, if the catalyst is the trimetallic Fe—Mo—Ni, the ratio of Fe:Mo:Ni can be 1:1:1, 3:2:1, 5:1:1, 10:2:1, and the like.

The metal, bimetal, or combination of metals can be used to prepare the catalyst as nanoparticles, preferably having defined particle size and diameter distribution. The catalyst can be prepared using the literature procedure described in Harutyunyan et al., NanoLetters 2, 525 (2002). Alternatively, the catalyst can be prepared by thermal decomposition of the corresponding metal salt added to a passivating salt, and the temperature of the solvent adjusted to provide the metal nanoparticles, as described in the co-pending and co-owned U.S. Pat. application Ser. No. 10/304,316, or by any other method known in the art. The size and diameter of the catalyst can be controlled by using the appropriate concentration of metal in the passivating solvent and by controlling the length of time the reaction is allowed to proceed at the thermal decomposition temperature. The metal salt can be any salt of the metal, and can be selected such that the melting point of the metal salt is lower than the boiling point of the passivating solvent. Thus, the metal salt contains the metal ion and a counter ion, where the counter ion can be nitrate, nitride, perchlorate, sulfate, sulfide, acetate, halide, oxide, such as methoxide or ethoxide, acetylacetonate, and the like. For example, the metal salt can be iron acetate (FeAc₂), nickel acetate (NiAc₂), palladium acetate (PdAc₂), molybdenum acetate (MoAc₃), and the like, and combinations thereof. The melting point of the metal salt is preferably about 5° C. to 50° C. lower than the boiling point, more preferably about 5° C. to about 20° C. lower than the boiling point of the passivating solvent. The solvent can be an ether, such as a glycol ether, 2-(2-butoxyethoxy)ethanol, H(OCH₂CH₂)₂O(CH₂)₃CH₃, which will be referred to below using the common name dietheylene glycol mono-n-butyl ether, and the like.

Catalysts having an average particle size of about 0.01 nm to about 20 nm, more preferably about 0.1 nm to about 3 nm and most preferably about 0.3 nm to 2 nm can be prepared. The catalysts can thus have a particle size of 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 nm, and up to about 20 nm. In another aspect, the catalysts can have a range of particle size, or diameter distribution. For example, the catalysts can have particle sizes in the range of about 0.1 nm and about 5 nm in size, about 3 nm and about 7 nm in size, or about 5 nm and about 11 nm in size.

The size and distribution of catalyst produced can be verified by any suitable method. One method of verification is transmission electron microscopy (TEM). A suitable model is the Phillips CM300 FEG TEM that is commercially available from FEI Company of Hillsboro, OR. In order to take TEM micrographs of the metal nanoparticles, 1 or more drops of the metal nanoparticle/passivating solvent solution are placed on a carbon membrane grid or other grid suitable for obtaining TEM micrographs. The TEM apparatus is then used to obtain micrographs of the nanoparticles that can be used to determine the distribution of nanoparticle sizes created.

In another method, the size of the catalyst particle, before and after being supported, can be determined using superconducting quantum interference device (SQUID) magnetometer, as described in the co-pending and co-owned application U.S. Ser. No. 10/992,275. SQUID magnetometers are commercially available from companies such as Biomagnetic Technologies Inc., San Diego, Calif. and Siemens AG of Germany, and include both single and multichannel devices that are capable of detecting magnetic fields at plurality of locations simultaneously. SQUID magnetometers generally comprise a superconducting pick up coil system and a detector system (the SQUID) which itself comprises one or two Josephson junctions inserted into a loop of superconducting wire. The magnetic flux within such loops is quantized and changes in the magnetic field experienced by the pick up coils cause a measurable change in the current flowing through the detector. The SQUID magnetometers are capable of measuring very low magnetic fields, for example, as low as 10⁻¹⁴ Tesla. The technique thus finds use in a variety of fields.

It has been found that the magnetization curves for catalysts having particle sizes of less than about 1.5 nm is paramagnetic, and the magnetization curves for particle sizes greater than about 2 nm is superparamagnetic, and greater than 4 nm is ferromagnetic Thus, in one aspect, the evolution of magnetic properties of the catalyst particles can be used for synthesis of the catalyst particles with the desired size. The metal/support material molar ratio can be varied if the magnetization curves observed in SQUID are not the same as the magnetization curves expected for the desired particle sizes. Thus, if the desired average particle size is about 2 nm or greater, such as between 2.1 nm and about 3 nm, then the variation of the metal/support material ratio is continued until the SQUID measurements provide magnetization curves indicating that the particles are superparamagnetic.

The catalysts, such as those formed by thermal decomposition described in detail above, and optionally characterized by SQUID, can then be supported on solid supports. The solid support can be alumina, silica, MCM-41, MgO, ZrO₂, aluminum-stabilized magnesium oxide, zeolites, or other oxidic supports known in the art, and combinations thereof. For example, Al₂O₃-SiO₂ hybrid support could be used. Preferably, the support is aluminum oxide (Al₂O₃) or silica (SiO₂). The oxide used as solid support can be powdered thereby providing small particle sizes and large surface areas. The powdered oxide can preferably have a particle size between about 0.01 μm to about 100 μm, more preferably about 0.1 μm to about 10 μm, even more preferably about 0.5 μm to about 5 μm, and most preferably about 1 μm to about 2 μm. The powdered oxide can have a surface area of about 50 to about 1000 m²/g, more preferably a surface area of about 200 to about 800 m²/g. The powdered oxide can be freshly prepared or commercially available.

In one aspect, the catalysts are supported on solid supports via secondary dispersion and extraction. Secondary dispersion begins by introducing particles of a powdered oxide, such as aluminum oxide (Al₂O₃) or silica (SiO₂), into the reaction vessel after the thermal decomposition reaction. A suitable Al₂O₃ powder with 1-2 μm particle size and having a surface area of 300-500 m²/g is commercially available from Alfa Aesar of Ward Hill, Mass., or Degussa, N.J. Powdered oxide can be added to achieve a desired weight ratio between the initial amount of the catalyst and the powdered oxide used to form the supported catalysts. Typically, the weight ratio can be between about 1:10 and about 1:50. For example, if 100 mg of iron acetate is used as the starting catalyst material, then about 320 to 480 mg of powdered oxide can be introduced into the solution. The weight ratio of catalyst to powdered oxide can be between about 1:10 and 1:20, such as, for example, 1:11, 1:12, 2:25, 3:37, 1:13, 1:14, 1:15, 1:16, 1:17, and the like.

As will be apparent to those of skill in the art, the supported catalyst thus prepared can be stored for later use. In another aspect, the catalyst can be previously prepared, isolated from the passivating solvent, and purified, and then added to a powdered oxide in a suitable volume of the same or different passivating solvent. The metal nanoparticles and powdered oxide can be homogenously dispersed, extracted from the passivating solvent, and processed to increase the effective surface area as described above. Other methods for preparing the metal nanoparticles and powdered oxide mixture will be apparent to those skilled in the art.

The supported catalysts thus formed can be used as a growth catalyst for synthesis of carbon nanotubes, nanofibers, and other one-dimensional carbon nanostructures using the chemical vapor deposition (CVD) process.

IV. Carbon Precursors

The carbon nanotubes can be synthesized using carbon precursors, such as carbon containing gases. In general, any carbon containing gas that does not pyrolize at temperatures up to 800° C. to 1000° C. can be used. Examples of suitable carbon-containing gases include carbon monoxide, aliphatic hydrocarbons, both saturated and unsaturated, such as methane, ethane, propane, butane, pentane, hexane, ethylene, acetylene and propylene; oxygenated hydrocarbons such as acetone, and methanol; aromatic hydrocarbons such as benzene, toluene, and naphthalene; and mixtures of the above, for example carbon monoxide and methane. In general, the use of acetylene promotes formation of multi-walled carbon nanotubes, while CO and methane are preferred feed gases for formation of single-walled carbon nanotubes. The carbon-containing gas may optionally be mixed with a diluent gas such as hydrogen, helium, argon, neon, krypton and xenon or a mixture thereof.

V. Synthesis of Carbon Nanotubes

The methods and processes of the invention provide for the synthesis of SWNTs having high quality. In one aspect of the invention, the catalyst supported on powdered oxides can be contacted with the carbon source at the reaction temperatures according to the literature methods described in Harutyunyan et al., NanoLetters 2, 525 (2002). Alternatively, the catalysts supported on the oxide powder can be aerosolized and introduced into the reactor maintained at the reaction temperature. Concurrently, the carbon precursor gas can be introduced into the reactor. The flow of reactants within the reactor can be controlled such that the deposition of the carbon products on the walls of the reactor is reduced. The carbon nanotubes thus produced can be collected and separated.

The catalysts supported on the oxide powder can be aerosolized by any of the art known methods. In one method, the supported catalysts are aerosolized using an inert gas, such as helium, neon, argon, krypton, xenon, or radon. Preferably argon is used. Typically, argon, or any other gas, is forced through a particle injector, and into the reactor. The particle injector can be any vessel that is capable of containing the supported catalysts and that has a means of agitating the supported catalysts. Thus, the catalyst deposited on a powdered porous oxide substrate can be placed in a beaker that has a mechanical stirrer attached to it. The supported catalyst can be stirred or mixed in order to assist the entrainment of the catalyst in the transporter gas, such as argon.

Thus, the nanotube synthesis generally occurs as described in the co-pending and co-owned application U.S. Ser. No. 10/727,707, filed on Dec. 3, 2003. An inert transporter gas, preferably argon gas, is generally passed through a particle injector. The particle injector can be a beaker or other vessel containing the growth catalyst supported on a powdered porous oxide substrate. The supported catalyst in the particle injector can be stirred or mixed in order to assist the entrainment of the supported catalyst in the argon gas flow. Optionally, the inert gas can be passed through a drying system to dry the gas. The argon gas, with the entrained powdered porous oxide, can then be passed through a pre-heater to raise the temperature of this gas flow to about 400° C. to about 500° C. The entrained supported catalyst is then delivered to the reaction chamber. A flow of methane or another carbon source gas and hydrogen can also be delivered to the reaction chamber. The typical flow rates can be 500 sccm for argon, 400 sccm for methane, and 100 sccm for He. Additionally, 500 sccm of argon can be directed into the helical flow inlets to reduce deposition of carbon products on the wall of the reaction chamber.

The temperature of the reaction chamber can be selected to be between about 300° C. and 900° C. The temperature can be selected such that it is preferably kept below the decomposition temperature of the carbon precursor gas. For example, at temperatures above 1000° C., methane is known to break down directly into soot rather than forming carbon nanostructures with the metal growth catalyst. Further, the temperature can be selected such that the formation of non-SWNT products, such as amorphous or disordered carbon, graphite, or multi-walled carbon nanotubes, is minimized. Carbon nanotubes and other carbon nanostructures synthesized in reaction chamber can then be collected and characterized.

In one aspect of the invention, the diameter distribution of the synthesized SWNTs is substantially uniform. Thus, about 90% of the SWNTs have a diameter within about 25% of the mean diameter, more preferably, within about 20% of the mean diameter, and even more preferably, within about 15% of the mean diameter. Thus, the diameter distribution of the synthesized SWNTs can be about 10% to about 25% within the mean diameter, more preferably about 10% to about 20% of the mean diameter, and even more preferably about 10% to about 15% of the mean diameter.

Carbon SWNTs can be synthesized with the yields ranging from about 4 wt % to about 15 wt % (wt % carbon relative to the iron/alumina catalyst). Analysis of transmission electron microscopy (TEM) images of SWNTs produced can be correlated with the size of the catalyst particles. For example, SWNTs can be produced as bundles having an average diameter of about 10 nm to about 15 nm when catalyst particle size is about 9 nm, having an average diameter of about 7 nm to about 12 nm when catalyst particle size is about 5 nm, and having an average diameter about 5 nm to about 10 nm when catalyst particle size is about 1 nm. The diameters estimated from TEM can be confirmed from the radial breathing modes observed by Raman Spectroscopy spectra, using multiple different laser excitations, such as, for example, λ=1064; 785; 614, 532, 514 and 488 nm.

The Raman spectra of SWNTs has three major peaks, which are the G-band at about 1590 cm⁻¹, D-band at about 1350 cm⁻¹, and the Radial breathing mode (RBM) at about 100-300 cm⁻¹. RBM frequency is proportional to an inverse of the diameter of SWNTs and can thus be used to calculate the diameter of the SWNT. Normally, a red shift in RBM peak corresponds to an increase in the mean diameter of SWNTs. The tangential mode G-band related to the Raman-allowed phonon mode E_(2g) can be a superposition of two peaks. The double peak at about 1593 and 1568 cm⁻¹ has been assigned to semiconductor SWNTs, while the broad Breit-Wigner-Fano line at about 1550 cm⁻¹ has been assigned to metallic SWNTs. Thus, G-band offers a method for distinguishing between metallic and semiconducting SWNTs. The D-band structure is related to disordered carbon, the presence of amorphous carbon, and other defects due to the sp²-carbon network. The ratio of the G-band to D-band in the Raman spectra (I_(G):I_(D) or G/D ratio) of SWNTs can be used as an index to determine the purity and quality of the SWNTs produced. Preferably, I_(G):I_(D) is about 1 to about 500, preferably about 5 to about 400, more preferably greater than about 7.

The reaction temperature and reaction duration can be varied to obtain SWNTs of high quality. For example, the reaction temperature can be from about 400° C. to about 900° C., preferably from about 500° C. to about 800° C., or more preferably from about from about 500° C. to about 750° C. The SWNTs produced at the initial setting can be tested for their quality, and the temperature adjusted such that the quality of SWNTs reaches the desired level. Preferably, the reaction temperature is less than about 750° C. The reaction duration can also be varied to obtain SWNTs of high quality. The reaction duration can be from about 1 min to about 180 min, preferably from about 10 min to about 120 min, more preferably from about 20 min to about 100 min. The quality of the SWNTs produced can be tested, and the reaction duration adjusted such that the quality of SWNTs reaches the desired level. Preferably, the reaction duration is less than about 90 min.

Using the methods and processes of the invention, high quality SWNTs with I_(G):I_(D) of about 5 to about 40 can be manufactured. The quality of the SWNTs can be controlled by controlling the particle size of the catalyst, the temperature of the reaction, and the duration of the reaction. The SWNTs thus produced are of greater purity and quality.

The carbon nanotubes and nanostructures produced by the methods and processes described above can be used in applications that include Field Emission Devices, Memory devices (high-density memory arrays, memory logic switching arrays), Nano-MEMs, AFM imaging probes, distributed diagnostics sensors, and strain sensors. Other key applications include: thermal control materials, super strength and light weight reinforcement and nanocomposites, EMI shielding materials, catalytic support, gas storage materials, high surface area electrodes, and light weight conductor cable and wires, and the like.

EXAMPLES

Below are examples of specific embodiments for carrying out the present invention. The examples are offered for illustrative purposes only, and are not intended to limit the scope of the present invention in any way. Efforts have been made to ensure accuracy with respect to numbers used (e.g., amounts, temperatures, etc.), but some experimental error and deviation should, of course, be allowed for.

Example 1

Preparation of the Supported Catalyst

Catalysts were prepared by impregnating support materials in metal salt solutions. The reaction time and metal salt/glycol ratio were changed to control the sizes of nanoparticles. In a typical procedure, FeAc₂ in methanol and MoAc₂ in methanol were used at a molar ratio of Fe:Mo:Al₂O₃ of 1:0.2:17. Under a nitrogen atmosphere, FeAc₂ and MoAc₂ were added to dietheylene glycol mono-n-butyl ether in the molar ratio of 1 mM:0.2 mM:20 mM. The reaction mixture was mixed under the nitrogen atmosphere using a magnetic stir bar, and heated under reflux for 90 minutes. The reaction mixture was then cooled to room temperature, and Al₂O₃ (17 mM) was added at once (molar ratio Fe:Mo:Al₂O₃=1:0.2:17). The reaction solution was stirred at room temperature for 15 minutes, and then heated to 150° C. for 3 hours. The reaction was cooled to 90° C. while flowing a stream of N₂ over the mixture to remove the solvent. A black film formed on the walls of the reaction flask. The black film was collected and ground with an agate mortar to obtain a fine black powder.

Example 2

Synthesis of Carbon Nanotubes

Carbon nanotubes were synthesized by using the experimental setup described in Harutyunyan et al., NanoLetters 2, 525 (2002). CVD growth of SWNTs with the catalysts produced above used methane as a carbon source (T=750° C., methane gas flow rate 60 sccm). In all cases, carbon SWNTs were successfully synthesized with the yields ˜4, 7 and 15 wt % (wt % carbon relative to the iron/molybdenum/alumina catalyst) for the catalysts. The single-walled carbon nanotubes thus produced were characterized using Raman spectra of carbon SWNTs using λ=785 nm laser excitation and by TEM.

The use of the supported catalyst Fe:Al₂O₃ having a molar ratio of 1:14.2 yielded I_(G):I_(D) ratio of 5, whereas, the use of the supported catalyst Fe:Al₂O₃ having a molar ratio of 1:14.2 yielded I_(G):I_(D) ratio of 8, and the use of the supported catalyst Fe:Mo:Al₂O₃ having a molar ratio of 1:0.2:16.9 yielded I_(G):I_(D) ratio of 21.5 (FIG. 1).

While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. All printed patents and publications referred to in this application are hereby incorporated herein in their entirety by this reference. 

1. A method for synthesizing carbon single-walled nanotubes (SWNTs), the method comprising: contacting supported metal catalyst with a carbon precursor gas, wherein SWNTs are synthesized with a ratio of G-band to D-band in Raman spectra (I_(G):I_(D)) of greater than
 5. 2. The method of claim 1, wherein the catalyst is selected from the group consisting of a Group V metal, a Group VI metal, a Group VII metal, a Group VIII metal, a lanthanide, and a transition metal, or mixtures thereof.
 3. The method of claim 2, wherein the catalyst is Fe and another metal selected from the group consisting of V, Nb, Cr, W, Mo, Mn, Re, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Ce, Eu, Er, Yb, Ag, Au, Zn, Cd, Sc, Y, and La or mixtures thereof.
 4. The method of claim 3, wherein the catalyst is Fe and Ni, Co, Cr, or Mo, and combinations thereof.
 5. The method of claim 4, wherein the catalyst is selected from the group consisting Fe—Mo, Fe—Co and Ni—Fe—Mo.
 6. The method of claim 1, wherein the support is a powdered oxide.
 7. The method of claim 6, wherein the powdered oxide is oxide is selected from the group consisting of Al₂O₃, SiO₃, MgO and zeolites.
 8. The method of claim 7, wherein the powdered oxide is Al₂O₃.
 9. The method of claim 1, wherein the catalyst and the support are in a ratio of about 1:1 to about 1:50.
 10. The method of claim 9, wherein the ratio is about 1:10 to about 1:20.
 11. The method of claim 1, wherein the carbon precursor gas is selected from the group consisting of carbon monoxide, methane, ethane, propane, butane, pentane, hexane, ethylene, acetylene, propylene, acetone, and methanol
 12. The method of claim 11, wherein the carbon precursor gas is methane.
 13. The method of claim 11, wherein the carbon precursor gas further comprises an inert gas and hydrogen.
 14. The method of claim 13, wherein the inert gas is argon, helium, nitrogen, hydrogen, or combinations thereof.
 15. The method of claim 1, wherein the ratio of I_(G):I_(D) is less than
 100. 16. The method of claim 15, wherein the ratio of I_(G):I_(D) is about 5 to about
 30. 17. The method of claim 15, wherein the ratio of I_(G):I_(D) is about 5 to about
 20. 18. A method for synthesizing carbon single-walled nanotubes (SWNTs), the method comprising: contacting metal catalyst on a support in a ratio of about 1:10 to about 1:20 with a carbon precursor gas, wherein SWNTs are synthesized with a ratio of G-band to D-band in Raman spectra (I_(G):I_(D)) of greater than 5, and wherein the metal catalyst is selected from the group consisting Fe—Mo, Fe—Co and Ni—Fe—Mo.
 19. The method of claim 18, wherein the support is a powdered oxide.
 20. The method of claim 19, wherein the powdered oxide is oxide is selected from the group consisting of Al₂O₃, SiO₃, MgO and zeolites.
 21. The method of claim 20, wherein the powdered oxide is Al₂O₃.
 22. The method of claim 18, wherein the carbon precursor gas is selected from the group consisting of carbon monoxide, methane, ethane, propane, butane, pentane, hexane, ethylene, acetylene, propylene, acetone, and methanol
 23. The method of claim 22, wherein the carbon precursor gas is methane.
 24. The method of claim 22, wherein the carbon precursor gas further comprises an inert gas and hydrogen.
 25. The method of claim 24, wherein the inert gas is argon, helium, nitrogen, hydrogen, or combinations thereof.
 26. The method of claim 18, wherein the ratio of I_(G):I_(D) is less than
 100. 27. The method of claim 26, wherein the ratio of I_(G):I_(D) is about 5 to about
 30. 28. The method of claim 26, wherein the ratio of I_(G):I_(D) is about 5 to about
 20. 29. The method of claim 18, wherein the reaction temperature is less than about 750° C. and the reaction duration is less than about 90 min. 